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NUMBER OF TOES
the hind-limbs must have preceded the fore-limbs in their
thorough adaptation to the cnrsorial mode of progression. In
the Mammalia the ankle-joint is always
what is termed cruro-tarsal, i.e. between
the ends of the limb-bones and the proximal
row of tar sals; not in the middle of the
tarsus as in some Sauropsida (reptiles arid
birds). The bones of the ankle are much
like those of the hand ; but there are
never more than two bones in the proxi- tr
mal row, which are the astragalus and the
calcaneum. The former is perhaps to be
looked upon as the equivalent of the
cuneiform and lunar together. Kut the
views as to the homologies of the tarsai
bones differ widely. IRelow these is the
navicular, regarded as a centrale. The
distal row of the tarsus has four bones,
three cuneiforms and a cuboid. Reduction FJGL 32.—Anterior aspect of
is effected by the soldering together of right femur of Rhinoceros
, . rt ,-, -r-f i ,-, (Rhinoceros indicus"). x i.
two cuneiiorms as in the Horse, by the -
fusion of the navicular and cuboid as in
the Deer. 2STo mammal has more than five
toes, and the number tends to become reduced in cursorial animals
(Rodents, Ungulates, Kangaroos).
Teeth.---The teeth of the Mammaliax differ from those of
other vertebrated animals in a number of important points.
These, however, entirely concern the form of the adult teeth,
their position in the mouth, and the succession of the series of
teeth. 3Developmentally and histologically there are no funda-
mental divergences from the teeth of vertebrates lower in the
In mammals, as for example in the Dog, the teeth consist of
three kinds of tissue—the enamel, the dentine, and the cement.
The enamel is derived from the epidermis of the mouth cavity,
and the two remaining constituents from the underlying dermis.
The teeth originate quite independently of the jaws, with which
they are later so intimately connected ; the independence of
origin being one of the facts upon which the current theory
1 Of. Tomes, A. M'a.nual of Dental Anatomy, 5th ed. London, 1898.
h, Head ; t, great trochan-
ter ; £', tlxird trocbanter.
(From Flower's Osteology.)